HC: use symbolic icons rather than a specific theme

https://bugzilla.gnome.org/show_bug.cgi?id=732521
HC: use symbolic icons for HC

- drop the custom rendered theme in favor of recolorable
  symbolic icons from Adwaita

https://bugzilla.gnome.org/show_bug.cgi?id=732521
background: move from here to gnome-backgrounds

- default bg now ships with gnome-backgrounds instead
  where the lock screen was.